Yeah I'd like to give you an expanded answer after reading the other responses.
The big bang happened roughly 13.8 billion years ago. Back then all matter was perfectly compacted into a super hot, super smooth, and super dense sphere. This in itself is amazing! If it wasn't in such a perfect state the matter would have simply collapsed on itself and formed a black hole. Instead it rapidly expanded and through billions of years has formed today's universe. This is supported with tons of evidence such as the acceleration of the universe and cosmic background radiation.
Now to figure out what happened before the big bang we can try to look even farther into our past. Sadly this is pretty much impossible since time and space were distorted beyond comprehension at the big bang. We can stop at this limit and think existence really is only 13.8 billion years old. Or, we can try going the opposite direction and see if our universe will ever have a big bang again. Not surprisingly this works.
Since entropy is always increasing and the universe is always expanding, eventually we will be nothing but an evenly spread out cloud of particles. At this moment it becomes a question of probability. What's the probability this cloud will randomly form a working PlayStation 4 complete with Killzone downloaded on the HDD? Infinitely small yet in this case time is also infinite so the PS4 should actually form. Now what are the chances that all matter in existence will perfectly align to form a super hot, super smooth, and super dense sphere? Again infinitely small yet over the course of infinite years it will happen. Alas we have conditions for another big bang.
So now we can logically apply this to the past and figure out pre big bang conditions. It was a 1 in a 9999999999... chance that the particles would align perfectly and they did.
*This is of course the most commonly accepted understanding and like all theoretical science could be wrong.
